1. Field of the Invention
The invention relates to finely divided silicas having narrow particle size distributions and a process for producing them.
2. Discussion of the Background
Silicas are produced industrially by precipitation of water glass or by combustion of silicon tetrachloride in a stream of H2/O2. The products obtained in this way usually do not have the desired particle size, or they must be subjected to subsequent drying. The product properties resulting from the production process, e.g. the specific surface areas determined by the BET or CTAB method should be changed as little as possible by the drying/milling step.
Customary apparatuses for comminution or milling of silicas are jet mills or impingement mills, and customary apparatuses for drying are spray dryers, rack dryers, rotary tube dryers or jet towers. Drying a silica suspension in a jet tower simultaneously effects particle formation.
The combination of the process steps: (a) preparation of the silica by precipitation or combustion, (b) drying and (c) milling, even when (b) and (c) coincide, e.g. in a jet tower, is not satisfactory for producing silica particles having a very narrow size distribution, so that a sieving or classification step frequently has to be carried out. This is costly.